Microwave-absorptive heat-generating body and method for forming a heat-generating layer in a microwave-absorptive heat-generating body

ABSTRACT

A sheet-like heat generation body for use in a microwave oven, which absorbs microwave and generates heat to irradiate food to be cooked. This heat generation body comprises a conductive film, which is made of a crystalline carbon as its principal component, and is formed on a sheet-like base material. The heat-generating body is prepared using a heat-resistant base material and an inorganic bonding agent applied to its surface. Specifically, a heat-resistant base is coated with a mixture of a microwave-absorbing and heat-generating material as its principal component and an agent for hardening an inorganic bonding agent to be applied later. After the mixed agent is dried, it is impregnated with the inorganic bonding agent.

TECHNICAL FIELD

The present invention relates to a microwave-absorptive heat-generatingbody which generates heat by absorbing energy of a microwave in anelectronic oven.

The present invention also relates to a method for forming aheat-generating layer in such microwave-absorptive heat-generating body.

BACKGROUND ART

An electronic oven is a device in which cooking is effected by makinguse of the nature that an irradiated microwave is absorbed by moleculesof water or the like contained in an article to be cooked, and it has amerit that generally cooking can be achieved in a short period of time.On the other hand, it cannot scorch food surfaces as is the case withexternal heating as by an oven, a gas range, an electric heater or thelike.

In order to overcome the above-mentioned shortcoming, a heat-generatingbody or a heat-generating container capable of scorching foods by makinguse of substance which generates heat by absorbing a microwave such asferrite, SiC, metal, barium titanate, etc., has been devised, and asintered body of ferrite, silicon carbide or the like, a pottery havingthe sintered body assembled therein, and furthermore, a body formed byapplying powder of these materials to a base material as a coating film,have been devised.

However, these heat-generating bodies involve many problems such thatits heat-generating property is insufficient, they cannot withstandthermal shocks caused by abrupt heat-generation, and they are expensiveand heavy in weight.

On the other hand, while a heat-generating sheet formed by applyingmetal vapor deposition of aluminium or the like onto a heat-resistingpaper sheet, a heat-resisting resin film or the like has been devised,it has a shortcoming that a stable heat-generating quantity can behardly assured because of the fact that it is necessary to make thethickness of vapor deposition considerably thin. It is hard to uniformlycontrol the thickness due to its thin film state, and itsheat-generating property would largely vary in the event that thethickness of the vapor deposition film should change.

A microwave-absorptive heat-generating body is formed of substanceshaving a heat-resisting property in view of its function. A principalmethod for manufacture thereof includes a method of forming a conductivethin film of Al, SnO₂, etc. on a surface of a heat-resisting basematerial through vapor-deposition; a method of obtaining theheat-generating body by sintering powder having a microwave-absorptiveheat-generating property such as ferrite, SiC, BaTiO₃, etc.; and amethod of fixedly securing powder having a microwave-absorptiveheat-generating property onto a surface of a heat-resisting basematerial by means of a heat-resisting organic bonding agent.

However, the above-mentioned heat-generating bodies produced throughvapor deposition and/or sintering necessitate a high temperature at thetime of manufacture and also result in a high cost in view ofinstallation or the like. Also, the method of fixedly securing materialhaving a microwave-absorptive heat-generating property by means of anorganic bonding agent is limited with respect to its heat-resistingtemperature.

In addition, although a method of fixedly securing the material byheating an inorganic bonding agent or adding a hardening agent to theinorganic bonding agent can be conceived in order to resolve theabove-mentioned problems, even in such method, in the case ofnecessitating to heat, rise of energy and installation costs result,while in the case of adding a hardening agent, degradation of theworking efficiency caused by decrease in the available time result. Ineither case, neither method is suitable to the case wheremass-production is required.

The present invention has been worked out in view of the above-mentionedpoint, and one object of the present invention is to provide asheet-like microwave-absorptive heat-generating body which is light inweight, flexible, excellent in a heat-generating property, and moreover,cheap.

Another object of the present invention is to provide a method forforming a microwave-absorptive heat-generating layer on a heat-resistingbase material by making use of an inorganic bonding agent at a lowtemperature, and moreover, under a sufficient available time.

DISCLOSURE OF INVENTION

According to the present invention, the above-mentioned former objectcan be achieved by the microwave-absorptive heat-generating bodiesdisclosed in the following:

(1) A microwave-absorptive heat-generating body, characterized in that aconductive coating film containing crystalline carbon as its principalcomponent is formed on a sheet-like base material.

(2) A heat-generating body as disclosed in paragraph (1) above,characterized in that the content in volume of crystalline carbon in theabove-described conductive coating film is 15% or more.

(3) A heat-generating body as disclosed in paragraph (1) or (2) above,characterized in that a microwave-permeable inorganic coating film layeris laminated between the above-mentioned coating film layer andsheet-like base material or on the upper surface of the above-mentionedcoating film layer.

(4) A heat-generating body as disclosed in paragraph (1), (2) or (3)above, characterized in that the thickness of the above-mentionedcoating film is 5 μm˜400 μm.

(5) A heat-generating body as disclosed in paragraph (1), (2), (3) or(4) above, characterized in that the above-mentioned coating film isformed in an array of divided small-area regions not continuous to oneanother, and the area of the continuous region is 5×5˜60×60 mm².

(6) A heat-generating body as disclosed in paragraph (5) above,characterized in that areas of the divided regions of theabove-mentioned coating film are varied depending upon its location onthe sheet-like base material.

The above-described heat-generating bodies would generate heat and wouldreach a high temperature through the process that the conductive coatingfilm containing carbon as its principal component and coated on thesheet-like base material absorbs a microwave radiated from a microwaverange, and external heating for scorching foods would be effected byconduction heat and radiation heat from such heat-generating bodies.

The sheet-like base material to be applied with the above-describedconductive coating film is a material capable of withstanding a hightemperature (200°˜400° C.) at the time of heat-generation, and so longas it is a microwave-permeable material, flame-resistant paper sheets,heat-resistant resin films, inorganic fiber paper sheets, etc. can beused widely, but in the case where the sheet reaches a considerably hightemperature, a sheet made exclusively of inorganic material is desirablebecause if an organic component is contained in the sheet base materialthere is a risk of generating a harmful gas, smoke and a nasty smell.Especially, glass fabrics are relatively cheap and also have aflexibility, and hence they are practically useful.

The above-described conductive coating film is principally formed ofcrystalline carbon, and it can be easily formed by coating a paintprepared by mixing carbon powder or carbon fibers with an inorganicbinder.

With regard to a method for coating, various methods such as screenprinting, letterpress printing, offset printing, etc. can be chosen, andthe method is not limited to a particular method.

While carbon has been heretofore well known as a heat-generatingsubstance, carbon used according to the present invention is what isgenerally called graphite, which is a laminated body composed of aparallel stack of networks each consisting of a large number of carbonatoms connected two-dimensionally in a regular hexagonal ring shape, andwhich is characterized in that it is crystalline carbon which isregularly laminated has excellent heat-resisting property, and is hardlyoxidized. In the so-called amorphous carbon such as carbon black,vitreous carbon or the like having a random layer structure in whichthere is no regularity in lamination, there exists a problem that itlacks a heat-resistant property and at the time of heat-generation itreacts with oxygen and results in smoke generation or deterioration ofproperties, and so, it is not suitable as a heat-generating substance.Also it has been heretofore known as a well-known fact that a goodproperty can be obtained by forming a heat-generating body so that asurface resistance value of the heat-generating body may become 10² ˜10⁵Ω, and in the present invention also, a good property can be revealedprovided that the surface resistance value falls in this range.

Between a resistance value and a specific resistance and a filmthickness is established the following relation:

    [Resistance value]=[Specific resistance]/[Film thickness]

The specific resistance of the coating film is different depending uponthe blending proportion of carbon in the paint to be coated, and tocomponents other than carbon such as an inorganic filler, a binder andthe like. If the percentage in volume of contained carbon is increased,the specific resistance of the coating film will become small, but onthe contrary, if the percentage by volume, of contained carbon isdecreased, the specific resistance of the film will become large.

In order to obtain a desired resistance value, adjustment could be doneto realize a film thickness matched with the specific resistance of thecoating film, and the range of this adjustment is about 5 μm˜1,000 μm.Since the coating film has a shortcoming in that if the thickness of thecoating film is increased, the flexibility of the entire sheet is lostand the inorganic coating film becomes fragile, it is desirable to makethe percentage in volume of contained carbon in the coating film to be15% or more and to make the film thickness to be 5 μm˜400 μm in view ofthe risk of damage during its handling. With regard to fillers otherthan carbon, provided that they are inorganic powder such as SiO₂, Al₂O₃, etc., they are not specifically limited to particular ones.Furthermore, for the purpose of making the sheet have flexibility, theabove-described coating film could be formed in an array of dividedsmall-area regions not continuous to one another rather than beingcoated over the entire surface of the sheet. In this modified case, notmerely it can be achieved to make the sheet have the flexibility, butalso a heat-generating quantity can be easily controlled so as to matchthe kind and amount of foods by decreasing a continuous area of acoating film in the case of suppressing its heat-generating propertyand, on the contrary, increasing it in the case of enhancing itsheat-generating quantity, paying attention to the fact that theheat-generating property and a continuous area of a coating film areproportional to each other and as the area of the divided regions of thesheet becomes larger its heat-generating property (absorbing efficiency)is improved.

Though the area of the divided regions is required to be 5×5 mm² ormore, because if the continuous area of the coating film is too small,its heat-generating quantity is so small that there is no effect, if itexceeds 60×60 mm², the flexibility of the sheet is deteriorated, and so,the scope of 5×5˜60×60 mm² is most suitable.

In addition, by applying a microwave-permeable inorganic coating filmbetween the above-described sheet base material and the conductivecoating layer as an intermediate layer, the sheet is made to have aheat-insulating effect, and thereby it is made possible to safely use iteven if the base material somewhat lacking a heat-resistant property.

Furthermore, although the conductive coating film principally consistingof crystalline carbon lacks a beautiful appearance, gives visuallysomewhat non-hygienic feeling as a body for use with foods and lacksexcellence in design because of its black color, its excellence indesign can be enhanced without degrading its properties by applying amicrowave-permeable inorganic coating film added with an inorganicpigment and the like onto the conductive coating film.

The above-mentioned inorganic coating film could be made of SiO₂, Al₂O₃, clay, glass, etc. and it is not specifically limited to particularmaterials.

The above-described latter object of the present invention can beachieved by the methods disclosed in the following:

(1) A method for forming a microwave-absorptive heat-generating layer,characterized in that at the time of forming a microwave-absoptiveheat-generating layer on a surface of a heat-resistant base material bymaking use of an inorganic bonding agent, after a mixture containing amicrowave-absorptive heat-generating substance as its principalcomponent and further containing at least one kind of hardening agentfor the aforementioned bonding agent has been applied onto theabove-mentioned base material, the above-described bonding agent isimpregnated in the aforementioned applied film and hardened.

(2) A method for forming a microwave-absorptive heat-generating layer asdisclosed in paragraph (1) above, characterized in that theaforementioned mixture contains Fe₃ O₄ as its principal component, andfurther the above-mentioned inorganic bonding agent is a phosphate groupbonding agent.

(3) A method for forming a microwave-absorptive heat-generating layer asdisclosed in paragraph (1) above, characterized in that theaforementioned mixture contains crystalline carbon as its principalcomponent, and further the aforementioned inorganic bonding agent is aphosphate group bonding agent.

(4) A method for forming a microwave-absorptive heat-generating layer asdisclosed in paragraph (1) above, characterized in that theaforementioned mixture contains crystalline carbon, Fe₃ O₄ and aluminasol, and further the aforementioned inorganic bonding agent is aphosphate group bonding agent.

At the time of forming a microwave-absorptive heat-generating layeraccording to the present invention, while the preliminarily appliedmixture is made to contain a microwave-absorptive heat-generatingsubstance and a hardening agent which is effective for an inorganicbinder to be impregnated later, in the event that the above-mentionedheat-generating substance is also provided with the effect of theaforementioned hardening agent, there is no need to newly add thehardening agent. In addition, besides the above-mentioned components,components essentially necessitated for forming an applied film such aswater, alcohol, a binder and the like are also contained in theabove-mentioned mixture.

Since an inorganic bonding agent that is effective for a hardening agentcontained in the above-described mixture is not contained in themixture, the shelf life of the mixture is greatly increased.

With regard to a method for forming the applied film, various methodssuch as spraying, dipping, printing, etc. can be conceived, anddepending upon necessity, different methods can be appropriatelyselected for use.

After formation of the applied film, it is dried under appropriateconditions, and thereafter it is impregnated with an inorganic bondingagent, and at this time also, the methods of spraying, dipping,printing, etc. can be appropriately selected for use.

While a hardening reaction commences within the applied film immediatelyafter impregnation, in the event that the effect is insufficient, theeffect can be improved by adding some heat. In addition, in the casewhere an organic component has been added into the mixture as a binderor the like, it is necessary to heat the mixture to remove it afterformation of the applied film or after impregnation of the inorganicbonding agent.

With regard to the inorganic bonding agent to be used, in view ofwater-proofness and bonding strength a phosphate group bonding agent ispreferable. Also as a hardening agent for this bonding agent, powders ofvarious hardening agents such as Fe₃ O₄, MgO, Al(OH)₃, activatedalumina, etc. are conceived, but a liquid state alumina sol is alsoeffective and it has a bonding effect in itself.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a plan view of a sheet-like heat-generating body according tothe invention;

FIG. 2 is a cross-section view of a heat-generating component partformed by supporting the above-mentioned sheet-like heat-generating bodyfrom a box-shaped support;

FIG. 3 is a cross-section view showing the state where theabove-described heat-generating component part is placed on a containerof refrigerated foods;

FIG. 4 is a cross-section view of a heat-generating component partmaking use of a sheet-like heat-generating body according to anotherprefered embodiment of the present invention; and

FIGS. 5 and 6 are plan views similar to FIG. 1, showing various modifiedembodiments of the arrangement and configuration of conductive coatingfilms on a sheet-like base material.

THE BEST MODE FOR CARRYING OUT THE INVENTION

Representative prefered embodiments of the present invention will beexplained with reference to the drawings.

FIGS. 1 to 6 are figures illustrating the representative preferedembodiments of the present invention.

In FIGS. 1 to 6, reference character S designates a sheet-likemicrowave-absorptive heat-generating body (hereinafter called"heat-generating sheet"), reference numeral 1 designates a sheet-likebase material, numeral 2 designates a conductive coating film, numeral 3designates foods or a container of foods, numeral 4 designates abox-shaped support, and numeral 5 designates a microwave-permeableinorganic coating film.

Example-1

A microwave-absorptive heat-generating sheet S was produced by carryingout printing on one surface of a sheet-like base material 1 consistingof glass fabrics with a mixture of black powder and a silica sol groupinorganic binder through a screen printing process while dividing theprinted area into a plurality of continuous coating film regions asshown in FIG. 1 in such manner that the size of each continuous coatingfilm region 2 is chosen to be a 25 mm□. Each discontinuous coating filmregion does not contact its adjacent regions. After being printed, thediscontinuous regions are baked at 200° C. for 1 Hr.

A heat-generating component part H was formed by sticking this sheet toa box-shaped support 4 made of a thick paper sheet into a structureshown in FIG. 2, then this component part was placed on a top surface ofa container 3' of a commercially available refrigerated cheese 3, andcooking by heating for about 7 minutes was effected by means of aconventional microwave range for domestic use.

In addition, with respect to a microwave-absorptive heat-generating bodyin which besides the aforementioned graphite powder, Al₂ O₃ powder wasadded as a filler and thereby a content of graphite was varied, also asimilar test was conducted.

The test results are shown in Table-1.

                  TABLE 1                                                         ______________________________________                                        Blending                 Resist-        Cooked                                Proportion                                                                              Graph-  Thin   ance   Specific                                                                              Con-                                  Graph-        ite     Film Value  Resistance                                                                            dition                              ite   Al.sub.2 O.sub.3                                                                      Vol. %  (μm)                                                                            R (Ω)                                                                          ρ (Ω/m)                                                                     Scorch                              ______________________________________                                        100   --      70      150   30    4.5 × 10.sup.-3                                                                 X                                                         70    75    5.3 × 10.sup.-3                                                                 X                                                         20   335    6.7 × 10.sup.-3                                                                 ⊚                    50    50      40      50   200    3.0 × 10.sup.-2                                                                 ⊚                                          70   550    3.9 × 10.sup.-2                                                                 ⊚                                          20   1650   3.3 × 10.sup.-2                                                                 ◯                       25    75      20      150    5 × 10.sup.4                                                                 7.5     Δ                                                   70   1.1 × 10.sup.5                                                                 7.7     X                                   10    90       9      150    2 × 10.sup.7                                                                 3.0 × 10.sup.3                                                                  X                                   ______________________________________                                    

As will be apparent from Table-1, resistance values falling in the rangeof 10² ˜10⁵ Ω, especially in the range of 10² ˜10³ Ω represent favorableresults.

The content percentage in volume of graphite is inversely proportionalto the specific resistance, and as the content lowers, an increase of aspecific resistance is observed. It is well understood that if thecontent proportion decreases, then in order to obtain the appropriateresistance values, a film thickness must be considerably increased, andfor a coating film having a graphite content proportion of 9%, athickness of 1˜10 m is necessitated to realize a resistance value of 10²˜10³ Ω.

Example-2

A conductive coating film having an area of divided individual coatingfilm region varied in the range of □3 mm˜□50 mm squares so as to have anequal total area of the entire coating film regions was formed on onesurface of each of several glass fabrics, then they were heated by meansof a conventional microwave range for domestic use, and their surfacetemperatures were measured by a radiation thermometer.

In this connection, at the time of heating by the microwave range, as aload 500 cc of water was simultaneously heated.

The results are shown in Table-2.

                  TABLE 2                                                         ______________________________________                                        Coating                                                                       Film Area        1 min.  2 min.                                               ______________________________________                                        3 × 3       80° C.                                                                         90° C.                                       5 × 5      100° C.                                                                        130° C.                                       7 × 7      215° C.                                                                        265° C.                                       10 × 10    350° C.                                                                        355° C.                                       20 × 20    385° C.                                                                        400° C.                                       30 × 30    500° C.                                                                        505° C.                                       50 × 50    550° C.                                                                        575° C.                                       ______________________________________                                    

It is seen that for an area of 5×5 mm² or less a heat-generatingquantity is small, and as an area increases a heat-generating quantitybecomes large.

Example-3

A paper napkin principally consisting of pulp was impregnated with waterglass, then subjected to acid treatment, washed by water, dried andsubjected to flame-proofing treatment. One surface of the prepared sheet1 was coated with a mixture consisting of 80 parts Al₂ O₃ powder, 20parts pearlite, an aluminium phosphate group binder and a hardeningagent to form a microwave-permeable inorganic coating film 5. The uppersurface of the formed coating film 5 was coated with a mixtureconsisting of Kish-graphite powder and an aluminium phosphate groupbinder to form a conductive coating film 2. Thereby a sheet having thestructure shown in FIG. 3 was produced. When commercially availablerefrigerated pizza pies (5 inches in diameter) 3 was placed on thissheet so as to come into contact with its upper surface and they werecooked for about 3 minutes by means of a microwave range, the crust wasscorched into light brown color, also good crispy feeling was obtained,and the pizza pie was properly cooked without being excessively heatedas a whole. On the other hand, no smoke nor no nasty smell was issued atall from the sheet.

Example-4

A heat-generating sheet was produced by applying a mixture consisting of30 parts graphite, 70 parts Fe₃ O₄ and a water glass group binder ontoone surface of a sheet 1 made of glass fabrics through a screen printingprocess so that the coating films may have a thickness of 200 μm and mayhave a large size at the central portion and successively reducing filmsizes towards its peripheral portion as shown in FIG. 4, and afterdrying, immersing the sheet in 20% aqueous acetic acid to convert waterglass into silica gel and form an insoluble coating film. When acommercially available pizza pie was placed on this sheet and cookingwas carried out in a microwave range, the entire surface was uniformlygiven crispiness and presented a good taste.

When same coating films were formed over the entire surface in a similarmanner as shown in FIG. 5, and a similar test was conducted, the centralportion was not scorched but somewhat wet, and crispiness was onlypresent at the peripheral portions.

Example-5

A sheet having a multiplicity of discontinuous areas of coating filmsthereon which were varied in size depending upon their locations asshown in FIG. 6, was produced through a process similar to that used inExample-4. Then a slice of bread was placed at the place where thecoating film areas are small, while a pizza pie was placed at the placewhere the coating film areas are large, and they were cookedsimultaneously.

Although a slice of bread is liable to be scorched as compared to apizza pie because of its light loading, they could both be appropriatelyscorched to a similar extent even if they were both cookedsimultaneously because the heat-generating rate of the sheet isdifferent between the respective sections.

Besides the above-mentioned example of use, this embodiment appears tobe effective upon simultaneously cooking different kinds of foods suchas are used in a lunch-box or the like.

As described above, the heat-generating sheet according to the presentinvention is light in weight, cheap, and excellent in a heat-generatingproperty. For its manufacturing process a procedure of printing can beused, and mass-production thereof is also easy. And so, it can be usedas a disposable sheet as inserted within a package jointly withcommercially available refrigerated foods or it can be integrated with apackage.

Since adjustment of a heat-generating property can be achieved bycontrolling not only the film thickness but also both the carbon contentand the film area in combination, design matched with foods can be doneeasily. Moreover, as it is also easy to vary the heat-generating ratedepending upon location, uniform cooking and selective cooking can becarried out.

Furthermore, since it is possible to give flexibility to the sheet, thesheet can be used in a deformed configuration so as to meet the shape offoods.

Example-6

Powders of ZrO₂ (mean particle diameter 10 μm), ZnO (mean particlediameter 5 μm), Fe₃ O₄ (-200 mesh), MgO (mean particle diameter 5 μm)and activated Al₂ O₃ (mean particle diameter 50 μm) were added withappropriate amounts of water, SiO₂ sol (Snowtex 30: made by NissanChemical Industry Co., Ltd.) and Al₂ O₃ sol (Aluminasol 200: made byNissan Chemical Industry Co., Ltd.) as a solvent. The mixture was coatedon a ZrO₂ plate of 50×50 mm² in a thickness of 0.5 mm, and then theplate was dried. Futhermore, these coating films were impregnated withwater glass (JIS 3) and aluminium phosphate (100 L made by Tagi ChemicalCo., Ltd.) by brushing, and thereafter they were dried at roomtemperature conditions. The obtained specimens were subjected to aboiling test for one hour, and the test results are shown in Table-3.

                  TABLE 3                                                         ______________________________________                                                               Impregnated Elution                                    Powders & Virtual      Inorganic   after                                      Volume Ratio                                                                              Solvent    Bonding Agent                                                                             Test                                       ______________________________________                                        ZrO.sub.2   water      water glass x                                          ZnO         ↑    ↑     ∘                              ZrO.sub.2 :ZnO = 1:1                                                                      ↑    ↑     ∘                              ZrO.sub.2   ↑    aluminium   x                                                                 phosphate                                              ZnO         ↑    ↑     x                                          ZrO.sub.2 :ZnO = 1:1                                                                      ↑    ↑     x                                          Fe.sub.3 O.sub.4                                                                          ↑    ↑     ∘                              ↑     SiO.sub.2 sol                                                                            ↑     ∘                              ↑     Al.sub.2 O.sub.3 sol                                                                     ↑     ∘                              ZrO.sub.2   SiO.sub.2 sol                                                                            ↑     x                                          ↑     Al.sub.2 O.sub.3 sol                                                                     ↑     ∘                              ZrO.sub.2 :MgO = 1:1                                                                      water      ↑     ∘                              ZrO.sub.2 :activated                                                                      ↑    ↑     ∘                              Al.sub.2 O.sub.3 = 1:1                                                        ______________________________________                                         ∘: No Elution                                                     X: Elution Observed                                                      

As will be obvious from Table-3, ZnO is effective as a hardening agentfor water glass, but for aluminium phosphate, Fe₃ O₄, MgO, activatedalumina and Al₂ O₃ sol are effective, and further, it is seen that thesehardening agents can also give water-proofness.

Example-7

After graphite powder having a mean particle diameter of 4 μm and MgOpowder having a mean particle diameter of 5 μm were mixed at a weightproportion of 35:65, an appropriate amount of water was added to themixture, and then the mixture was sprayed on a dish of Φ200 made ofcordierite to form a coating film. After the film was dried at roomtemperature, the above-described aluminium phosphate was impregnatedinto the applied coating film likewise through spraying after drying thefilm at room temperature, it was further dried at 200° C. for 30minutes, and thereby a microwave-absorptive heat-generating body wasobtained.

As made, the thickness of the heat-generating layer was 10 μm, and itsresistance value was 100˜1000 Ω. After this dish-shaped heat-generatingbody was subjected to a boiling test for one hour, neither elution norchange of a resistance value was observed. In addition, when this dishwas heated by a microwave for 1 minute in a microwave range of 500 W andits surface temperature was measured by a radiation thermometer, themeter indicated 260° C., but neither generation of cracks nor pealingcaused by thermal shocks was observed in the heat-generating layeritself.

Example-8

Graphite powder having a mean particle diameter of 4 μm and Fe₃ O₄powder of -200 mesh were mixed at a weight proportion of 15:85. Anappropriate amount of the above-described Al₂ O₃ sol was added, andthereby an ink for use in screen printing was prepared. After this inkwas applied to a surface of glass fabrics through a screen printingprocess in the pattern shown in FIG. 1, it was dried at roomtemperature, and further the above-described aluminium phosphate wasimpregnated in this printed layer in a similar pattern. Thereafter, theprinted layer was dried at 200° C. for 1 minute, and thereby a microwaveabsorptive heat-generating layer was obtained.

As made, the thickness of the heat-generating layer was 50 μm and itsresistance value was 200˜500 Ω. After this sheet was subjected to aboiling test for one hour, neither elution nor change of a resistancevalue was observed.

Furthermore, a support made of a paper sheet was provided at theperipheral portion of this heat-generating sheet, and thereby amicrowave-absorptive heat-generating component part H as shown in FIG. 2was obtained. As shown in the same figure, this component part wasplaced above a commercially available refrigerated cheese andmicrowave-heating was effected for 8 minutes in a 500 W microwave range.Then, the surface of the cheese was appropriately scorched, and itsinterior had a sufficiently cooked condition.

Example-9

A surface of a net made of steel having a wire diameter of 1 mm, anouter diameter of 160 mm and a mesh pitch of 15 mm was subjected to acidtreatment to make it appropriately rough, and the net was dipped in aslurry consisting of -200 mesh Fe₃ O₄ powder and Al₂ O₃ sol. Thereafterit was dried at a room temperature, and then it was further dried at200° C. for 1 hour, and after this net was sufficiently impregnated withthe above-described aluminium phosphate by brush-painting, it was driedat a room temperature, then it was dried at 200° C. for 1 hour, andthereby a net-like microwave-absorptive heat-generating body wasobtained. When this net was subjected to a boiling test for 1 hour,elution was not observed.

Still further, when a commercially available 6-inch refrigerated pizzapie was placed on this net and the net was heated by a microwave for 3minutes in a 500 W microwave range, the pizza pie could be cooked withits crust portion scorched. Also, anomalies such as cracks, pealings andthe like were not observed in the heat-generating body after cooking.

As described in detail above, by making use of the method according tothe present invention, a microwave-absorptive heat-generating layerhaving a heat-resisting property can be obtained at a low temperature ina short period of time. Moreover, according to the present invention,the workability is excellent because the reactions of the hardeningagent and the bonding agent would occur only within the applied film.

At this time, by selecting a water-absorptive porous body such ascordierite, glass fabrics or the like as a heat-resistant base material,further shortening of a drying time as well as improvements in a bondingstrength between a base material and a heat-generating layer can beachieved.

Although the subject heat-generating layer is required to havewater-proofness in the case where the heat-generating body comes intocontact with foods because generally moisture is contained in the foodsto be cooked in a microwave range, this can be overcome by selecting awater-proof inorganic bonding agent as represented by aluminiumphosphate.

INDUSTRIAL APPLICABILITY

The microwave-absorptive heat-generating body according to the presentinvention can be utilized for externally heating and cooking foods byabsorbing a microwave generated in a microwave range and generating heatat the time of microwave-range cooking.

The method for forming a heat-generating layer according to the presentinvention can be utilized for producing a microwave-absorptiveheat-generating body as described above.

We claim:
 1. A microwave-absorptive heat-generating body comprisinga conductive coating film disposed on a heat resistant sheet-like base material, wherein said conductive coating film comprising: a crystalline carbon as its principal component, a filler selected from the groups consisting of at least one of silica and alumina, and a hardened reaction product of an inorganic bonding agent comprising a phosphate bonding agent, with a hardening agent comprising Fe₃ O₄.
 2. A heat-generating body as claimed in claim 1, wherein said conductive coating film is comprised of at least 15% by volume of crystalline carbon.
 3. A heat-generating body as claimed in claim 1 further comprising a microwave-permeable inorganic film layer laminated on a surface of said coating film disposed away from said sheet-like heat resistant base material.
 4. A heat-generating body as claimed in claim 1 wherein the thickness of said coating film is 5 μm to 400 μm.
 5. A heat-generating body as claimed in claim 1 wherein said coating film occupies a multiplicity of discontinuous regions on a surface of said heat resistant sheet-like base material, and wherein the area of each discontinuous region, respectively, is about 5×5 to 60×60 mm².
 6. A heat-generating body as claimed in claim 5, wherein said areas of said discontinuous regions are dissimilar.
 7. A heat-generating body as claimed in claim 1 further comprising a microwave-permeable inorganic film layer laminated between said coating film and said heat resistant sheet-like base material.
 8. A method for forming a layer microwave-absorptive, heat-generating material on a surface of a heat-resistant base material comprising:disposing a mixture, comprising crystalline carbon, as a microwave-absorptive heat-generating substance as its principal component and further containing at least one kind of hardening agent, comprising Fe₃ O₄ as its principal component, onto a surface of said heat resistant base material to form at least one layer of said mixture on said base material, thereafter impregnating an inorganic bonding agent, comprising a phosphate group bonding agent hardenable by said hardening agent, into said mixture, and then hardening said inorganic bonding agent by reaction with said hardening agent to form said microwave absorptive, heat-generating layer.
 9. A method for forming a microwave-absorptive heat-generating layer as claimed in claim 7, wherein said mixture comprises crystalline carbon, Fe₃ O₄ and alumina sol.
 10. A method as claimed in claim 7 further comprising disposing a microwave permeable inorganic film between said layer and said heat resistant base material.
 11. A method as claimed in claim 7 further comprising disposing a microwave permeable inorganic film on a surface of said layer directed away from said heat resistant base material.
 12. A method as claimed in claim 7 further comprising applying a plurality of layers of said heat-generating material to discontinuous regions of said surface of said heat-resistant base material.
 13. A method as claimed in claim 12 comprising applying at least said plurality of layers at said discontinuous regions heat-generating regions and plurality of layers are similarly sized and shaped.
 14. A method for forming a layer microwave-absorptive heat-generating material as claimed in claim 7, wherein said mixture comprises crystalline carbon, Fe₃ O₄ and alumina sol, and wherein said inorganic bonding agent comprises a phosphate group bonding agent.
 15. A method as claimed in claim 7 further comprising disposing a microwave permeable inorganic film between said layer and said heat-resistant base material.
 16. A method as claimed in claim 7 further comprising disposing a microwave permeable inorganic film on a surface of said layer directed away from said heat-resistant base material.
 17. A method as claimed in claim 7 further comprising applying a plurality of layers of said heat generating material to discontinuous regions of said surface of said heat-resistant base material.
 18. A method as claimed in claim 17, wherein said discontinuous regions and plurality of layers are similarly sized and shaped. 